# -*- encoding=utf-8 -*-
print('----------------------方法1--------------------------')


# 方法1,实现__new__方法
# 并在将一个类的实例绑定到类变量_instance上,
# 如果cls._instance为None说明该类还没有实例化过,实例化该类,并返回
# 如果cls._instance不为None,直接返回cls._instance
class Singleton(object):
    def __new__(cls, *args, **kw):
        if not hasattr(cls, '_instance'):
            orig = super(Singleton, cls)
            cls._instance = orig.__new__(cls, *args, **kw)
        return cls._instance


class MyClass(Singleton):
    a = 1


one = MyClass()
two = MyClass()

two.a = 3
print(one.a)
# 3
# one和two完全相同,可以用id(), ==, is检测
print(id(one))
# 29097904
print(id(two))
# 29097904
print(one == two)
# True
print(one is two)
# True

print('----------------------方法2--------------------------')


# 方法2,共享属性;所谓单例就是所有引用(实例、对象)拥有相同的状态(属性)和行为(方法)
# 同一个类的所有实例天然拥有相同的行为(方法),
# 只需要保证同一个类的所有实例具有相同的状态(属性)即可
# 所有实例共享属性的最简单最直接的方法就是__dict__属性指向(引用)同一个字典(dict)
# 可参看:http://code.activestate.com/recipes/66531/
class Borg(object):
    _state = {}

    def __new__(cls, *args, **kw):
        ob = super(Borg, cls).__new__(cls, *args, **kw)
        ob.__dict__ = cls._state
        return ob


class MyClass2(Borg):
    a = 1


one = MyClass2()
two = MyClass2()

# one和two是两个不同的对象,id, ==, is对比结果可看出
two.a = 3
print(one.a)
# 3
print(id(one))
# 28873680
print(id(two))
# 28873712
print(one == two)
# False
print(one is two)
# False
# 但是one和two具有相同的（同一个__dict__属性）,见:
print(id(one.__dict__))
# 30104000
print(id(two.__dict__))
# 30104000

print('----------------------方法3--------------------------')


# 方法3:本质上是方法1的升级（或者说高级）版
# 使用__metaclass__（元类）的高级python用法
class Singleton2(type):
    def __init__(cls, name, bases, dict):
        super(Singleton2, cls).__init__(name, bases, dict)
        cls._instance = None

    def __call__(cls, *args, **kw):
        if cls._instance is None:
            cls._instance = super(Singleton2, cls).__call__(*args, **kw)
        return cls._instance


class MyClass3(object):
    __metaclass__ = Singleton2


one = MyClass3()
two = MyClass3()

two.a = 3
print(one)
# 3
print(id(one))
# 31495472
print(id(two))
# 31495472
print(one == two)
# True
print(one is two)
# True

print('----------------------方法4--------------------------')


# 方法4:也是方法1的升级（高级）版本,
# 使用装饰器(decorator),
# 这是一种更pythonic,更elegant的方法,
# 单例类本身根本不知道自己是单例的,因为他本身(自己的代码)并不是单例的
def singleton(cls, *args, **kw):
    instances = {}

    def _singleton():
        if cls not in instances:
            instances[cls] = cls(*args, **kw)
        return instances[cls]

    return _singleton


@singleton
class MyClass4(object):
    a = 1

    def __init__(self, x=0):
        self.x = x


one = MyClass4()
two = MyClass4()

two.a = 3
print(one.a)
# 3
print(id(one))
# 29660784
print(id(two))
# 29660784
print(one == two)
# True
print(one is two)
# True
one.x = 1
print(one.x)
# 1
print(two.x)
# 1



